The present invention relates to the preparation of water-soluble cross-linked cationic polymers, and methods for preparing such polymers with high molecular weights. The present invention also relates to methods for using such polymers to coat substrates and to separate suspended particle from an aqueous medium.
Cationic polymers have been used extensively in water treatment, paper making, mineral processing, petroleum recovery, textile dyeing, cosmetics and pharmaceuticals. Among the most important and extensively used cationic polymers are the quaternary ammonium polymers of diallyldimethylammonium chloride (DADMAC). In many cases, it is known that high molecular weight cationic polymers are more effective in such end uses.
Polymerization of DADMAC is typically carried out in aqueous solution using a free radical initiator such as a persulfate salt. Several approaches have been tried to increase the molecular weight of DADMAC polymers, including polymerization with added inorganic salts, polymerization in oil-in-water emulsions or suspended droplets, and addition of cross-linkers during polymerization. These methods are well known to those skilled in the art.
U.S. Pat. No. 4,222,921 discloses that the use of a diallylamine salt other than hydrohalide markedly speeds up the polymerization rate using ammonium persulfate (APS) as initiator. The conversion of monomer to polymer was substantially greater when the diallylamine salt polymerized was a salt of a strong acid (e.g., sulfuric acid) other than hydrohalide acids (e.g., hydrochloric acid). It was speculated that the halide ion acted as a chain transfer agent and a chain terminator.
Jaeger et al., Macromol. Sci. Chem., A21(5) 593 (1984) reported that persulfate could oxidize chloride ion to produce chlorine radical which then would terminate polymerization and decrease molecular weight. They obtained relatively high MW polyDADMAC using an azo initiator instead of persulfate.
U.S. Pat. No. 4,742,134 discloses that increased polymerization rate and molecular weight can be obtained using fluoride salts with persulfate initiator. Halide salts other than fluoride (e.g. NaCl) did not accelerate polymerization or increase molecular weight.
U.S. Pat. No. 4,439,580 nevertheless demonstrated that use of ammonium persulfite initiator with added NaCl salt in inverse emulsion polymerization also gave high MW polylDADMAC. The narrow pH range (8.0 to 10.5) and added salts used in the inverse (water-in-oil) emulsion polymerization were claimed to be critical elements for successful polymerization.
Use of cross-linking or branching agents in polymerization is another way to produce high MW cationic polymers. Polymerization with cross-linking agents can give high MW as well as structured polymers. A highly branched polyDADMAC can have better efficacy than a linear one of similar MW in certain types of applications. U.S. Pat. No. 3,544,318 discloses that branched polyDADMAC works better than linear polyDADMAC for electroconductive paper because the branched polymer imparts superior properties to the electroconductive paper substrate, preventing solvent diffusion into the paper.
U.S. Pat. No. 3,968,037 also showed that cationic polymers made by inverse emulsion polymerization with cross-linking and branching agents had surprisingly high effectiveness as flocculants and for the treatment of activated sewage sludge. Polyolefinic unsaturated compounds, such as tri- and tetra-allylammonium salts or methylene bisacrylamide, were used as cross-linking agents. Only ineffective products were obtained by solution polymerization with a cross-linking agent.
EP 264,710 also disclosed that highly branched, water soluble polyDADMAC made by solution polymerization worked better as a flocculent or as a defoaming agent for breaking oil-in-water emulsions. Highly branched polyDADMAC was made by adding 0.1 to 3.0 mole % of cross-linking co-monomer such as methyltriallylammonium chloride (MTAAC) or triallylamine hydrochloride (TAAHCl) during the polymerization of DADMAC, beginning after monomer conversion had reached 25% to 90%. A completely gelled product was obtained when the MTAAC was added all at once in the beginning.
The amount of epihalohydrin added is only partially reacted in the final resins to maintain water solubility. If allowed to react fully, the poly(diallylamine)-epihalohydrin resins will gel and become water insoluble. Only epihalohydrin is disclosed as a cross-linker for diallylamine polymers. Other polyfunctional compounds which can be used to cross-link the diallylamine polymers are not mentioned.
U.S. Pat. Nos. 3,700,623 and 3,833,531 teach the making of acid-stabilized poly(diallylamine)-epihalohydrin resins. Polymers of diallylamine were first prepared through radical polymerization using a radical initiator. The polymer of diallylamine was then reacted with an epihalohydrin, usually epichlorohydrin, at a temperature of from about 30 to 80.degree. C. and a pH of from 7 to 9.5 in aqueous solution. When the viscosity measured on a 20% to 30% solid solution reached a desired viscosity range (A to E on the Gardner-Holdt scale), the product was diluted with water to below 15% solids. The obtained resin has a tendency to gel on standing. The resin solution is thus stabilized against gelation by addition of sufficient water-soluble acid (e.g. Hcl) to maintain the pH at about 2. The acid-stabilized poly(diallylamine)-epichlorohydrin resins are reactivated prior to use by addition of a base (e.g. NaOH) to adjust pH to above 7.
The polymers of diallylamine which can be reacted with an epihalohydrin for making the poly(diallylamine) epihalohydrin resins include homopolymers and copolymers of a reactive diallylamine monomer. The reactive diallylamine monomers include secondary diallylamines and tertiary monoalkyldiallylamines such as methyldiallylamine. The poly(diallylamine)- epihalohydrin resins are polymers having more than 5 mole % of recurring units derived from the reactive diallyamine monomer.
The diallylamine units in the polymers are capped with epihalohydrin. The added epihalohydrin is mostly half reacted to minimize cross-linking and maintain water solubility. Because of the high content of diallylamine and epihalohydrin used, water-insoluble gel products are obtained if the added epihalohydrin is allowed to react fully. Therefore, the resins need to be stabilized by addition of acid. Although soluble, these capped prepolymers are not yet cross-linked.
The half-reacted epihalohydrin entities of the alkaline curing resins impart epoxy functionality for cross-linking reactions after being reactivated by addition of alkaline base prior to use. These polymers are insoluble after cross-linking.
U.S. Pat. Nos. 4,354,006; 4,419,498; and 4,419,500 teach a process for making poly(DAA-Epi)polymers by reacting a DAA polymer first with an allyl halide and then with hypohalous acid to convert the allyl substituents to halohydrin moieties.
JP 6,108,382 discloses another way to make poly(diallylamine)-epihalohydrin polymers. A diallylamine-epihalohydrin (DAA-Epi) halo salt monomer is first prepared by reacting diallylamine with an epihalohydrin (typically epichlorohydrin) and then neutralizing with a halo acid (typically HCl). The DAA-Epi tertiary amine salt monomer is then polymerized using a radical initiator. The obtained poly(diallyamine)-epihalohydrin polymer is disclosed to provide excellent wet color fastness to a cellulose-based fiber dyed with a direct dye or a reactive dye.
U.S. Pat. No. 5,147,411 discloses a method to prepare the DAA-Epi monomers (3-halo-2-hydroxypropyl)diallylamine and (2,3-epoxypropyl)diallylamine, and their quaternary ammonium salts. The quaternary ammonium DAA-Epi salts are prepared by reacting a DAA-Epi tertiary amine with an alkylsulfonate. The DAA-Epi quaternary ammonium salts are used directly in treating cellulose fiber material for improved color yield and wet fastness of dyeing.
U.S. Pat. No. 4,341,887 discloses that the reaction product of diallylamine and epichlorohydrin (3-chloro-2-hydroxypropyl)diallylamine (a DAA-Epi monomer), can be converted to N,N-diallyl-3-hydroxy-azetidinium chloride (DAA-Epi azetidinium monomer) by heating in presence of water. Removal of the solvent (water) by distillation or freeze drying causes the DAA-Epi azetidinium monomer to reconvert to the linear, non-quaternary N-3-chloro-2-hydroxypropyl-N,N-diallylamine. (3-chloro-2-hydroxypropyl)diallylamine is not stable for long periods of time and dimerizes to 2,5-bis(diallylaminomethyl)-p-dioxane. The azetidinium ring remains intact in the polymers obtained by free radical polymerization of the DAA-Epi azetidinium monomer. .sub.1 H NMR and .sup.13 C NMR was used to identify the azetidinium ring in the monomer and the polymers. The homo- and co-polymers of N,N-diallyl-3-hydroxyazetidinium are useful for demulsification, flocculation and floatation in water treatment.
U.S. Pat. No. 4,537,831 discloses use of diallylamine polymers as agents for cross-linking chlorine-containing polymers such as copolymers of vinyl chloride and ethylene.
The alkaline-curable polymer systems have high diallylamine and epihalohydrin content (&gt;5 mole %) for required high cross-link density in their end uses as resins.
In order to prepare cationic polymers of high molecular weight by solution polymerization, it is usually necessary to use solutions with high concentrations of the cationic monomer. During the course of the polymerization, the viscosity of the reaction medium increases to very high levels. Poor mixing and heat transfer in these highly viscous solutions limits the obtainable molecular weight of the cationic polymers. Accordingly, there remains a need for high molecular weight water-soluble cationic polymers.